1. Field of the Invention
This invention relates to the field of secondary surveillance radar (SSR) systems, which are systems designed to operate in conjunction with primary radar systems for precisely locating aircraft by transmitting information to them and processing their replies. More particularly, this invention relates to an SSR system in which monopulse processing is used to obtain angle estimation of azimuth bearing of aircraft.
By monopulse processing is meant techniques which determine aircraft azimuth on a pulse-by-pulse basis so as to permit highly accurate azimuth estimates to be made on a single reply per scan. Such techniques permit a very high degree of accuracy in determining azimuth. Moreover, monopulse techniques permit secondary surveillance radar (SSR) operation at a greatly reduced pulse repetition frequency compared to that required by more conventional azimuth measurement techniques.
2. Background Information
For a general description of a secondary surveillance radar system, reference may be made to a report entitled "Surveillance Performance Measurements of the SSR Mode of the Discrete Address Beacon System" by Vincent A. Orlando and Paul R. Drouilhet, MIT Lincoln Laboratory. That reference describes a system involving the use of a multibeam antenna, and the target off-boresight azimuth is determined by the relative magnitude of the received signal strength in the Difference (.DELTA.) and Sum (.SIGMA.) channels of the system. In the particular context involved, boresight is defined as the precise angle at which the radio frequency antenna pattern is pointing at any instant.
A second system, which is known to the present inventors provides that the magnitudes or amplitudes of the signals in Difference (.DELTA.) and Sum (.SIGMA.) channels serve as a means of estimating how far the target return is off-boresight. Since the ratio of the Difference (.DELTA.) signal to the Sum (.SIGMA.) signal is the point of interest, it is convenient to deploy log amplifiers. However, if the signal magnitude only is being used, there is ambiguity as to whether the target is left or right of boresight. In the implementation shown in this second system, means are provided for resolving the ambiguity. Thus, samples of the carrier signal are taken from the Sum, (.SIGMA.) and Difference (.DELTA.) channels. These signals are then compared on a phase basis to determine whether the target is left or right. This second reference also describes a system in which a third channel, called the Omni (.OMEGA.) or control channel, is similarly processed by deployment of a log amplifier. A signal is provided in this third channel to suitable components to indicate that the target is in the main beam of the antenna, rather than being a side or back lobe response which requires suppression.
The four signals involved in this second system, that is the Sum (.SIGMA.) video, the Difference (.DELTA.) video, sign, and control video are further processed to determine if the target return is indeed from the main beam and, if so, to estimate how far left or right it is from boresight. The circuitry of this particular system is straightforward, provided the log amplifiers are well matched, and further provided that the Difference (.DELTA.) amplitude stays well above thermal noise levels. In practice this is not achieved, particularly when one considers maximum range (the weakest signal returns) and targets on or near boresight. Under the latter conditions, the Difference (.SIGMA.) signal will be perhaps 20-30 db lower in magnitude than the Sum (.epsilon.) signal.
A third system of particular interest to the present invention is that described in the report FAA-RD-79-111 entitled "The Transportable Measurements Facility (TMF) System Description" by R. R. La Frey et al. The system of this third reference operates in such a way that the Sum (.SIGMA.) and Difference (.DELTA.) channel outputs from the antenna are combined to produce two signals, i.e. (.SIGMA.+j.DELTA. and .DELTA.+j.SIGMA., which will always have the same magnitude but which will differ in angle depending on the value of the Difference (.DELTA.) signal.
The system of this third reference is sometimes referred to as a one-half angle monopulse processor. In such system, the combined signals .SIGMA.+j.DELTA. and .DELTA.+j.SIGMA. are linearly amplified after mixing to produce an IF signal, typically having a frequency of 60 megahertz. At this point in such system, samples from these two channels are combined again to produce resultant signals, which are the Sum (.SIGMA.) and Difference (.DELTA.) signals. The Sum (.SIGMA.) term is again split and part is passed through a log amplifier, and part provides the reference for the one-half angle processor phase detectors. The inputs to the phase detectors are passed through limiters so that the amplitude outputs are functions of phase difference only. Each phase detector provides an independent measurement of one-half of the angle between the two channels; hence the name, one-half angle processor.
As was the case with the first two systems cited, an Omni (.OMEGA.) or control channel is also provided, and its signal is mixed, filtered, and amplified by means of a log amplifier, the output signal thereof being compared with a log amplifier output from the Difference (.DELTA.) channel. The purpose of this arrangement is to provide a signal to the overall system indicating that the signal is indeed from the main beam, and not a side or back lobe response from the antenna.
It is apparent that the second system earlier described is less complex than the third system just described above. However, its performance is relatively poor and this largely comes about from the fact that the signals processed in the Sum (.SIGMA.) and Difference (.DELTA.) channels are greatly different in amplitude as the target varies left and right of boresight. Moreover, the Difference (.DELTA.) signal from the log amplifier will be immersed in the thermal noise level, particularly near boresight. This makes this particular system susceptible to sign errors when the angle of target deviation is near the boresight.
A further attribute of the third system described above is that the processor used to estimate the angle off-boresight in this case is much simpler than is the case in the first and second systems, since the circuit provides a signal whose amplitude is a function directly of off-boresight displacement.
It will thus be appreciated that although the three known systems already described have various merits and advantages, they do not provide an efficient, simple and cost effective SSR system.
Accordingly, it is a primary object of the present invention to provide such a system; more specifically, to provide the performance available from one-half angle processing without the expense entailed with separate linear and limiting amplifiers.
Another object is to avoid the need for a separate log amplifier to provide a log video Sum (.SIGMA.) signal.
Yet another object is to avoid the requirement for precise tracking of the log video characteristic. This is possible in the system of the invention because phase information alone is used for azimuth angle estimate.
A further object is to provide a system in which the .epsilon.+j.DELTA. and .DELTA.+j.SIGMA. channels are each provided with a log amplifier. These log amplifiers enable, in combination with phase detectors in each of the channels, a determination of the azimuth angle estimate, while the log amplifiers operate at the same signal level independent of the target position in the main beam. (.SIGMA.
Yet a further object is to attain the simplicity of the amplitude processing technique that is inherent in the systems described in the first and second references previously noted, while attaining the superior performance inherent in the one-half angle processing signal described in the third reference noted.